It has been suggested that in addition to dust production, \(^{16}\) O/\(^{18}\) O ratios could be a second difference between RCB and dLHdC stars. [1]} and [2]} noted that dLHdC stars have \(^{16}\) O/\(^{18}\) O \(<1\)  and most RCB stars have \(^{16}\) O/\(^{18}\) O \(>1\) . However, this analysis is based on a small sample of only two dLHdC and five RCB stars Note that the star HD 175893 was originally classified as a dLHdC star but is now known to be an RCB star based on an IR excess found by [3]}. Both dLHdC stars in the sample have \(^{16}\) O/\(^{18}\) O \(<1\) , while three of the five RCB stars have \(^{16}\) O/\(^{18}\) O \(>1\) . [4]} measured the ratios for six additional RCB stars using medium resolution spectra and found that they had \(^{16}\) O/\(^{18}\) O\(>1\) . However, additional measurements of these ratios for dLHdC stars were not possible in the past because of the five dLHdC stars known at the time, only three had temperatures cold enough to show the CO bands [2]}, [6]}, [4]}.
